1. Field of the Invention
The present invention relates to a keysheet module used in small electronic devices such as mobile phones.
2. Description of Related Art
Small electronic devices such as mobile phones have conventionally built in a keysheet module having parts, such as dome-shaped contact switches and LEDs, mounted on a flexible printed circuit board (FPC board). The keysheet module is arranged on the underside of a keypad formed integral with a plurality of keytops (see Japanese Patent Disclosure Nos. 2002-170457 and 2001-35305).
FIG. 6 to FIG. 8 illustrate an example construction of a conventional keysheet module. In these figures, a keysheet module 1 comprises a flexible printed circuit board 2 formed by glass epoxy resin, a plurality of dome-shaped depression switches 3 provided on an upper surface of the flexible printed circuit board 2, fine wiring patterns 4 formed on the flexible printed circuit board 2, electronic parts arranged appropriately between the wiring patterns 4, such as light emitting diodes 5 as a backlight illuminating the keytops and resistors 6, and a sheet 80 fixedly bonded to the upper surface of the flexible printed circuit board 2. The sheet 80 has air holes 70 that are formed in the sheet and connect openings 71, formed at positions corresponding to movable contacts 13 of the depression switches 3, to allow air trapped in the movable contacts 13 to escape (see FIG. 7). This arrangement is made to provide the user with a good clicking feel as he or she depresses the depression switches 3. The sheet 80 also functions as a white reflection plate that reflects light emitted from the light emitting diodes 5.
Over the movable contacts 13, a switch holding sheet 90 is fixedly bonded to the upper surface of the sheet 80. The switch holding sheet 90 is a transparent sheet that covers the movable contacts 13 for appropriate positioning and holding. Further, the keysheet module 1 has a flexible connector 10 provided with power supply and signal wires for the supply of electricity to the keysheet module 1 and for the transfer of signals to and from the module.
The dome-shaped depression switches 3, as shown in FIG. 8, are each made up of a first fixed contact 11 formed on the flexible printed circuit board 2, a roughly ring-shaped second fixed contact 12 surrounding the first fixed contact 11, and the dome-shaped movable contact 13 connected to the second fixed contact 12. The first fixed contact 11 is connected to a wiring pattern not shown and the second fixed contact 12 is connected to a wiring pattern 4a. The movable contact 13 is formed by an elastic thin metal such as stainless steel. In FIG. 8, reference number 14 represents a solder resist layer printed over the flexible printed circuit board 2 to protect the wiring pattern 4a. 
Next, an operation of each of the depression switches 3 with the above construction will be explained. The movable contact 13 of the depression switch 3 has its lower peripheral portion in contact with the second fixed contact 12 and out of contact with the first fixed contact 11 at all times. In other words, the depression switch 3 is open or off. In this state, when the movable contact 13 is pressed down from above the switch holding sheet 90, it is elastically depressed and comes into contact with the first fixed contact 11, turning on the depression switch 3. At this time, air trapped in the movable contact 13 is pushed out of the air holes 70 in the sheet 80, giving the depression switch 3 a good clicking feel. Then, when the depressing force is released from the movable contact 13, the movable contact 13 returns to its original state by its own elastic force, turning off the depression switch 3.
This conventional keysheet module 1, however, has a problem that since the air trapped in the movable contact 13 is released through the air holes 70 in the sheet 80, the keysheet module requires the sheet 80, increasing the number of parts and therefore the number of assembly steps of the keysheet module 1.
Japanese Patent Disclosure No. 2001-35305 discloses a construction in which an adhesive layer is provided instead of the sheet 80, with air holes formed in the adhesive layer. However, this construction has a drawback that the air holes are easily deformed, making the amount of air discharged unstable, which may in turn degrade the clicking feel when the dome-shaped depression switch is pressed.